Continuous connection between the two record classes

Determine whether the Haldane and trivial record classes can be joined continuously, thereby establishing whether a record has phases in the full sense rather than only distinct classes.

Background

The paper constructs a transition between a Haldane-class record and a trivial-class record using symmetry-protected sectors with different bond representations. In the exhibited example, covariance prevents the sectors from mixing, so their leading transfer eigenvalues cross at a first-order transition where the fixed-point structure becomes nonunique.

It remains unresolved whether the two classes can instead be connected continuously in a suitable symmetry-preserving setting. The authors identify this question as bearing on whether records possess genuine phases or merely classifications into distinct classes.

References

The transition here is first order because covariance keeps the sectors apart, and whether two classes can be joined continuously would say whether a record has phases in the full sense or only classes.

Forbidden Subspaces in Quantum State Smoothing  (2609.17307 - Kam et al., 15 Sep 2026) in Conclusion